merge-ort/merge-recursive: do report errors in `merge_submodule()`

merge-ort/merge-recursive: do report errors in `merge_submodule()`

In 24876ebf68b (commit-reach(repo_in_merge_bases_many): report missing
commits, 2024-02-28), I taught `merge_submodule()` to handle errors
reported by `repo_in_merge_bases_many()`.

However, those errors were not passed through to the callers. That was
unintentional, and this commit remedies that.

Note that `find_first_merges()` can now also return -1 (because it
passes through that return value from `repo_in_merge_bases()`), and this
commit also adds the forgotten handling for that scenario.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>